1. Field of the Invention
The present invention concerns a method to determine a three-dimensional target image data set showing at least one partial region of interest of an acquisition region, of the type wherein the image data of the three-dimensional target image data set are reconstructed from two-dimensional projection images acquired from various projection directions. The invention also concerns an x-ray device for implementing such a method.
2. Description of the Prior Art
Methods to reconstruct a three-dimensional image data set from two-dimensional projection images that were acquired from various projection directions are widely known. For example, it is possible to process the two-dimensional projection images in iterative methods or methods of filtered back-projection in order to determine a three-dimensional reconstruction image data set. Such procedures are mostly used in fields in which spatial information is also desired. In addition to computed tomography (CT), methods similar to computed tomography are known for x-ray devices with a C-arm and for tomosynthesis. The methods of computed tomography have also been used for some time in other x-ray systems (such as flat panel detector x-ray systems) in interventional and diagnostic environments to enable a high-quality volumetric imaging.
The achievable three-dimensional image quality within the acquisition region (field of view, FOV) depends on the reconstruction method that is used, but the image quality is principally theoretically limited by the selection of the acquisition parameters particularly the acquisition trajectory, the acquisition positions and the resolution of the x-ray detector that is used, thus the projection image count and the pixel size, as well as the x-ray dose that is used.
Consequently, in the prior art it is typical to select these acquisition parameters appropriately in the design of a measurement protocol in order to acquire sufficient 3D image information for a meaningful evaluation. For example, it is known to acquire approximately 400 projection images using a 2×2 binning for applications at C-arm x-ray devices with the goal of a good low contrast resolution. For tomosynthesis screening of a female breast, a very high pixel resolution (consequently no binning) is typically selected in the acquisition. The acquisitions deliver the desired high image quality across the entire acquisition region. A disadvantage of such methods similar to computed tomography is the high radiation exposure to which the patient is exposed because of the acquisition of the large number of projection images in the acquisition region.
It is often unnecessary to acquire the entire three-dimensional acquisition region (field of view) with the same image quality. Often, only partial regions of interest need to be evaluated more precisely, for example tumors in the tissue, cartilage between joint bones or fractures. These partial regions should be shown with as high a quality as possible, but a poorer quality would be sufficient for the surroundings. A problem, however, is that before beginning the acquisition, the precise position of these partial regions of interest in the image volume is frequently unknown.
In order to be able to examine a partial region of interest more precisely, methods have been proposed that address the aforementioned problem. If a three-dimensional image data set is calculated from an acquired sequence of projection images, and the three-dimensional image data set has approximately consistently high quality over the entire acquisition region, then if only a partial region thereof is of interest, a (digital) zoom function can be used in the visualization in order to show the partial region of interest enlarged. In this way details can be visually emphasized, but a gain in image information is not provided by a (digital) zoom.
In an article by D. Kolditz et al., “Volume-of-interest (VOI) imaging in C-arm flat-detector CT for high image quality at reduced dose”, Med. Phys. 37 (6), June 2010, Pages 2719-2730, it is proposed to initially implement a low-dose scan from which a three-dimensional reconstruction image data set is reconstructed in which the partial region of interest is localized. A second scan subsequently takes place along the same scan path (acquisition trajectory) but with a higher x-ray dose and with collimation at the partial region of interest so that the radiation is focused only on the localized partial region of interest (often also designated as ROI). The final image result—thus the target image data set—is then reconstructed from the projection images of the second scan.